This invention generally relates to systems and methods for constructing and using holographic elements, and more particularly to systems and methods for recording optical matched filters and using those filters at a multitude of wavelengths.
In the construction of holographic optical elements, a first construction beam is projected such that it is incident upon a recording medium. As is well known, the recording medium can be a photographic emulsion, dichromated gelatin, a photopolymer, and the like, and can be coated or mounted on a suitable substrate such as a glass plate, thin film, and the like. Simultaneously, and from the same source of coherent electromagnetic radiation, which preferably is a laser, a second construction beam is directed at an angle so that it is incident upon the recording medium such that it overlaps the first construction beam at the medium. The result of the overlapping input beams on the recording medium is an optical interference pattern which is recorded in the medium as an amplitude or phase distribution of closely spaced lines. If the first input beam is normal to the plane of the recording medium, the spacing, b, between lines formed in the lens is determined by the equation: EQU b=.lambda./sin.theta. (1)
where .lambda. is the wavelength of the construction beams, and .theta. is the angle between the plane of the recording medium and the second construction beam.
In use, should a holographic lens be illuminated with a collimated beam of radiation, an off-axis focus will be achieved. If the beam remains collimated but the wavelength is changed, a second off-axis focus having a different offset angle and focal distance than the first is obtained. This result is the consequence of the fact that physically a hologram is basically a highly complex diffraction grating. The angle, .theta., and focal length, F, of a collimated beam of light having wavelength .lambda. that is dispersed by a holographic lens are given by the equations: EQU sin.theta.=m .lambda./b (2)
and EQU F=(.lambda.c/.lambda. F.sub.c ( 3)
where .lambda..sub.c and F.sub.c are the wavelength and focal length of the beam used to construct the hologram, and b is the spacing of the line array formed in the holograph. .lambda. and F are referred to as the playback wavelength and focal length respectively.
Thus, the relationship between the dispersion angles, .theta..sub.o and .theta..sub.1, of the collimated light beams of two different wavelengths, .lambda..sub.o and .lambda..sub.1, dispersed by a holographic lens is given by the equation: ##EQU1## and the relationship between the focal distances, F.sub.o and F.sub.1, of those two light beams is given by the equation: ##EQU2##
Matched filters are one type of holographic element that are used in optical correlator systems to detect the presence of a selected target in a scene or a field of view. To construct a matched filter, one of the collimated construction beams, referred to as the signal beam, is spatially modulated by passing it through an image of the selected target. The two construction beams then combine at the matched filter plane to produce a diffractionn pattern unique to the selected target. When a matched filter is used in an optical correlator system, a collimated light beam is passed through a selected view and then transmitted to the matched filter. The output of the matched filter is a light beam directed to an inverse transform lens. If the selected target is not present in the view, the output of the matched filter is relatively weak and diffused, and that output remains diffused as it passes through the inverse transform lens. However, if the suspected target is present in the submitted view, the light traversing the matched filter becomes collimated, and the inverse transform lens brings the output beam from the matched filter to a focus.
A light sensitive detector is located at the focal point of the inverse transform lens, and when light is focused on that detector, an output signal is produced. This output signal is used to trigger some type of device, depending upon the apparatus in which the target recognition system is used. Such a device might be a simple alarm or a complex guidance system, for example.
It is often advantageous to fabricate a matched filter at one wavelength and to use the filter at a second wavelength. For example, some images are recorded best in a matched filter at a wavelength in the blue light spectra and played back best at a wavelength in the red light spectra. In addition, in some situations, when a matched filter is operated at the same wavelength at which it was fabricated, the operating light signal has a tendency to alter the image formed in the matched filter. This tendency is substantially reduced if the matched filter is operated at a wavelength different from the wavelength used to fabricate the filter.
Heretofore, individual optical systems have not been designed to manufacture or operate matched filters readily at multiple wavelengths. Systems having this flexibility would have particular advantages in remote locations such as on satellites where it is difficult, if not practically impossible, to reposition the various elements of an optical system in any significant way to operate the system at different wavelengths. Such a system would also have significant utility in a laboratory or similar setting, since it would eliminate the need, and the time required, to alter the system substantially to operate matched filters at multiple wavelengths.